Microelectromechanical gyroscope with calibrated synchronization of actuation and method for actuating a microelectromechanical gyroscope

ABSTRACT

A gyroscope includes a body, a driving mass, which is mobile according to a driving axis, and a sensing mass, which is driven by the driving mass and is mobile according to a sensing axis, in response to rotations of the body. A driving device forms a microelectromechanical control loop with the body and the driving mass and maintains the driving mass in oscillation with a driving frequency. The driving device comprises a frequency detector, which supplies a clock signal at the frequency of oscillation of the driving mass, and a synchronization stage, which applies a calibrated phase shift to the clock signal so as to compensate a phase shift caused by components of the loop that are set between the driving mass and the control node.

BACKGROUND

1. Technical Field

The present disclosure relates to a microelectromechanical gyroscopewith calibrated synchronization of actuation and to a method foractuating a microelectromechanical gyroscope.

2. Description of the Related Art

As is known, the use of microelectromechanical systems (MEMS) has becomeprogressively widespread in various sectors of technology and hasyielded encouraging results especially for providing inertial sensors,microintegrated gyroscopes, and electromechanical oscillators for a widerange of applications.

MEMS of this type are usually based on microelectromechanical structurescomprising at least one movable mass connected to a fixed body (stator)by springs and movable with respect to the stator according topre-determined degrees of freedom. The movable mass is moreover coupledto the fixed body via capacitive structures (capacitors). The movementof the movable mass with respect to the fixed body, for example onaccount of an external stress, modifies the capacitance of thecapacitors; from this it is possible to trace back to the relativedisplacement of the movable mass with respect to the fixed body andhence to the force applied. Vice versa, by supplying appropriate biasingvoltages, it is possible to apply an electrostatic force to the movablemass to set it in motion. In addition, to provide electromechanicaloscillators, the frequency response of inertial MEMS structures isexploited, which is typically of the second-order low-pass type.

Many MEMS (in particular, all electromechanical oscillators andgyroscopes) include driving devices that have the task of maintainingthe movable mass in oscillation.

A first type of known solution envisages supplying, in open loop,periodic excitation at the resonance frequency of the MEMS structure.The solution is simple, but also far from effective, because theresonance frequency is not known with precision on account of theineliminable dispersions in the processes of micromachining ofsemiconductors. In addition, the resonance frequency of each individualdevice can vary over time, for example, on account of temperaturegradients or, more simply, on account of aging.

Feedback driving circuits have then been proposed, based upon the use ofsigma-delta modulators. Circuits of this type are undoubtedly moreeffective than the previous ones in stabilizing the oscillation of themovable mass at the real resonance frequency and in suppressingdisturbance.

However, various stages are employed for filtering, decimation, andfurther processing of the bitstream supplied by the sigma-deltamodulator. For this reason, currently available feedback drivingcircuits are complex to produce, cumbersome and, in practice, costly.

In addition, it should be considered that gyroscopes have a complexelectromechanical structure, which comprises two masses that are movablewith respect to the stator and are coupled to one another so as topresent a relative degree of freedom. The two movable masses are bothcapacitively coupled to the stator. One of the movable masses isdedicated to driving (driving mass) and is kept in oscillation at theresonance frequency. The other movable mass (sensing mass) is driven inthe oscillatory motion and, in the case of rotation of themicrostructure with respect to a pre-determined axis with an angularvelocity, is subject to a Coriolis force proportional to the angularvelocity itself. In practice, the sensing mass operates as anaccelerometer that enables sensing of the Coriolis acceleration.

For enabling actuation and providing an electromechanical oscillator inwhich the sensor performs the role of frequency-selective amplifier,with transfer function of a second-order low-pass type and high meritfactor, the driving mass is equipped with two types of differentialcapacitive structures: driving electrodes and driving-detectionelectrodes. The driving electrodes have the purpose of sustainingself-oscillation of the movable mass in the direction of actuation,through electrostatic forces generated by the spectral component of thenoise at the mechanical resonance frequency of the driving mass. Thedriving-detection electrodes have the purpose of measuring, through thetransduced charge, the position of translation or rotation of thesensing mass in the direction of actuation.

The U.S. Pat. No. 7,305,880 describes a system for controlling thevelocity of oscillation of the gyroscope, comprising a differentialsense amplifier, a high-pass amplifier, and an actuation and controlstage, operating in a continuous-time mode.

The U.S. Pat. No. 7,827,864 describes an improvement of the foregoingcontrol system, in which the control loop comprises a low-pass filter inorder to reduce the offset and the effects of parasitic components andcouplings by operating on the overall gain and phase of the feedbackloop.

These systems, albeit operating frequently in a satisfactory way, may,however, undergo improvement as regards area occupation. These systemssynchronize the read and control circuits precisely in order to preservethe advantages deriving from the use of microstructures with driving andsensing masses not electrically insulated from one another (inparticular on account of the technological difficulties in providing theinsulation, which render the manufacturing processes considerably morecomplex and costly). For this purpose, phase-locked-loop (PLL) circuitsare normally used, which have, however, a far from negligible impact interms of area occupation, as well as of consumption levels, and includeexternal filtering components. In addition, at the start and uponwaking-up from low-consumption (the so-called “power-down”)configurations or from conditions of loss of synchronism, the PLLcircuits may have transients even of several hundreds of millisecondsbefore completing phase locking The consequent delay in the response canbe very detrimental in certain applications.

BRIEF SUMMARY

Some embodiments of the present disclosure provide amicroelectromechanical gyroscope and a method for actuating amicroelectromechanical gyroscope that will make it possible to overcomethe limitations described.

According to the present disclosure, a microelectromechanical gyroscopeand a method for actuating a microelectromechanical gyroscope areprovided, as claimed in claim 1 and claim 19, respectively.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a better understanding of the disclosure, some embodiments thereofwill now be described, purely by way of non-limiting example and withreference to the attached drawings, wherein:

FIG. 1 is a simplified block diagram of a microelectromechanicalgyroscope according to one embodiment of the present disclosure;

FIG. 2 is a top plan view of an enlarged detail of the gyroscope of FIG.1;

FIG. 3 is a top plan view of a further enlarged detail of the gyroscopeof FIG. 1;

FIG. 4 is a graph regarding signals used in the gyroscope of FIG. 1;

FIG. 5 is a more detailed block diagram of a first component of thegyroscope of FIG. 1;

FIG. 6 is a more detailed block diagram of a second component of thegyroscope of FIG. 1;

FIG. 7 is a more detailed block diagram of a part of the secondcomponent of FIG. 6;

FIG. 8 is a graph regarding signals used in the gyroscope of FIG. 1; and

FIG. 9 is a simplified block diagram of an electronic systemincorporating a microelectromechanical gyroscope according to oneembodiment of the present disclosure.

DETAILED DESCRIPTION

FIG. 1 shows as a whole a microelectromechanical gyroscope 1, whichcomprises a microstructure 2, made of semiconductor material, a drivingdevice 3, a read generator 4, and a read device 5.

The microstructure 2 is made of semiconductor material and comprises abody 6, a driving mass 7, and at least one sensing mass 8. Forsimplicity, in the embodiment illustrated herein reference will be madeto the case of a uniaxial gyroscope, in which a single sensing mass 8 ispresent. What is described hereinafter applies, however, also to thecase of multiaxial gyroscopes, which comprise two or more sensing massesor systems of sensing masses, for detecting rotations according torespective independent axes.

The driving mass 7 is elastically constrained to the fixed structure 6so as to be able to oscillate about a resting position according to atranslational or rotational degree of freedom. The sensing mass 8 ismechanically coupled to the driving mass 7 so as to be driven in motionaccording to the degree of freedom of the driving mass 7 itself. Inaddition, the sensing mass 8 is elastically constrained to the drivingmass 7 so as to oscillate in turn with respect to the driving mass 7itself, with a respective further degree of freedom, in response torotational movement of the body 6.

In the embodiment described herein, in particular, the driving mass 7 islinearly movable along a driving axis X, while the sensing mass 8 ismovable with respect to the driving mass 7 according to a sensing axisY, that is perpendicular to the driving axis X.

It is understood, however, that the type of movement (whethertranslational or rotational) allowed by the degrees of freedom and thearrangement of the driving and sensing axes may vary according to thetype of gyroscope. With reference to the movements of the driving mass 7and of the sensing mass 8, moreover, the expression “according to anaxis” will be indifferently used in relation to movements along an axisor about an axis, according to whether movements allowed to the massesby the respective degrees of freedom are translational (along an axis)or else rotational (about an axis). Likewise, the expression “accordingto a degree of freedom” will be indifferently used in relation totranslational or rotational movements, as allowed by the degree offreedom itself.

In addition, the driving mass 7 (with the sensing mass 8) is connectedto the body 6 so as to define a resonant mechanical system with aresonance frequency ω_(R) (according to the driving axis X).

The driving mass 7 (FIG. 2) is capacitively coupled to the body 6through driving units 10 and feedback-sensing units 12. The capacitivecoupling is of a differential type.

In greater detail, the actuation units 10 comprise first and secondfixed driving electrodes 10 a, 10 b, which are anchored to the body 6and extend substantially perpendicular to the driving direction X, andmovable driving electrodes 10 c, which are anchored to the driving mass7 and are also substantially perpendicular to the driving direction X.The movable driving electrodes 10 c are comb-fingered and capacitivelycoupled with respective first fixed driving electrodes 10 a and secondfixed driving electrodes 10 b. In addition, the first and second fixeddriving electrodes 10 a, 10 b of the actuation units 10 are electricallyconnected to a first driving terminal 13 a and to a second drivingterminal 13 b, respectively, of the microstructure 2. Furthermore, ashas been mentioned, the coupling is of a differential type. In otherwords, in each actuation unit 10 a movement of the driving mass 7 alongthe driving axis X causes the capacitance between the movable drivingelectrode 10 c and one of the fixed driving electrodes 10 a, 10 b toincrease. The capacitance between the movable driving electrode 10 c andthe other of the fixed driving electrodes 10 a, 10 b decreases insteadaccordingly.

The structure of the feedback-sensing units 12 is similar to that of theactuation units 10. In particular, the feedback-sensing units 12comprise first and second fixed sensing electrodes 12 a, 12 b, anchoredto the body 6, and movable sensing electrodes 12 c, anchored to thedriving mass 7 and comb-fingered and capacitively coupled withrespective first fixed sensing electrodes 12 a and second fixed sensingelectrodes 12 b. In addition, the first and second fixed sensingelectrodes 12 a, 12 b of the feedback-sensing units 12 are electricallyconnected respectively to a first feedback-sensing terminal 14 a and asecond feedback-sensing terminal 14 b of the microstructure 2.

Hence, in practice, the driving mass 7 is coupled to the drivingterminals 13 a, 13 b through differential driving capacitances C_(D1),C_(D2) and to the sensing terminals 14 a, 14 b through feedback-sensingdifferential capacitances C_(FBS1), C_(FBS2).

The sensing mass 8 is electrically connected to the driving mass 7,without interposition of insulating structures. Consequently, thesensing mass 8 and the driving mass 7 are at the same potential. Thesensing mass 8 is moreover capacitively coupled to the body 6 throughsignal-sensing units 15 (FIG. 3). More precisely, the signal-sensingunits 15 comprise third and fourth fixed sensing electrodes 15 a, 15 b,anchored to the body 6, and movable sensing electrodes 15 c, anchored tothe sensing mass 8 and arranged between respective third fixed sensingelectrodes 15 a and fourth fixed sensing electrodes 15 b. Also in thiscase, the capacitive coupling is of a differential type, but it isobtained through parallel-plate electrodes, which are perpendicular tothe sensing direction Y. In addition, the third and fourth fixed sensingelectrodes 15 a, 15 b of the signal-sensing units 15 are electricallyconnected, respectively, to a first signal-sensing terminal 17 a and toa second signal-sensing terminal 17 b of the microstructure 2. Inpractice, the sensing mass 8 is coupled to the signal-sensing terminals17 a, 17 b through signal-sensing differential capacitances C_(SS1),C_(SS2).

With reference again to FIG. 1, the driving device 3 is connected to thedriving terminals 13 a, 13 b and to the feedback-sensing terminals 14 a,14 b of the microstructure 2 so as to form, with the driving mass 7, anoscillating microelectromechanical loop 18, with control of position ofthe driving mass 7. In greater detail, the driving device 3 comprises acharge amplifier 20, a band-pass filter 21, a low-pass interpolatorfilter 22, a variable-gain amplifier 23, a controller 24, a comparator25, and a digital phase-shifter module 26. In addition, an oscillator27, a temperature-compensation module 28, a synchronization stage 29,and a timing generator 30 are used for supplying timing signals for thedriving device 3, for the read generator 4, and for the read device 5.

The microelectromechanical loop 18 is of a hybrid type. The chargeamplifier 20 is, in fact, of the switched-capacitor type and isconfigured for operating in discrete time, whereas the low-pass filter22 and the variable-gain amplifier 23 operate in continuous time. Theband-pass filter 21 carries out time-discrete-to-time-continuousconversion. In addition, the charge amplifier 20 defines a detectioninterface for detecting the position x of the driving mass 7 withrespect to the driving axis X. The remaining components of the drivingdevice 3 co-operate for controlling, on the basis of the position x ofthe driving mass 7, the amplitude of oscillation of themicroelectromechanical loop 18, in particular the amplitude ofoscillation of the driving mass 7, and maintain it close to a referenceamplitude. The reference amplitude is, in particular, determined by areference voltage V_(REF), which is supplied to the controller 24.

The charge amplifier 20, which is of a fully differential type and hasinputs respectively connected to the first and second feedback-sensingterminals 14 a, 14 b, defines a detection interface for detecting theposition x of the driving mass 7 with respect to the driving axis X. Thecharge amplifier 20 receives differential feedback charge packetsQ_(FB1), Q_(FB2) from the feedback-sensing terminals 14 a, 14 b of themicrostructure 2 and converts them into feedback voltages V_(FB1),V_(FB2), indicative of the position x of the driving mass 7. In thisway, the charge amplifier 20 carries out a discrete-time reading of theposition x of the driving mass 7.

The band-pass filter 21 is cascaded to the charge amplifier 20 andintroduces a phase shift that is as close as possible to 90° and in anycase comprised in the interval 90°±40°. In one embodiment, the band-passfilter 21 comprises a sample-and-hold circuit and is moreover configuredso as to carry out a first low-pass filtering. Phase-shifted feedbackvoltages V_(FB1)′, V_(FB2)′ supplied by the band-pass filter 21 are thusdelayed and attenuated with respect to the feedback voltages V_(FB1),V_(FB2). The phase-shifted feedback voltages V_(FB1)′, V_(FB2)′ presentbasically step-like variations.

The low-pass filter 22 is arranged downstream of the band-pass filter21, is a fully differential filter at least of the second order, andsupplies filtered feedback voltages V_(FB1)″, V_(FB2)″ which arecontinuously variable in time. The cut-off frequency of the low-passfilter 22 is selected in such a way that the frequency of oscillation ofthe microelectromechanical loop 18 (in particular of the driving mass7), hereinafter referred to as driving frequency ω_(D), is included inthe passband and in such a way that the phase of the useful signalindicating the position x of the driving mass 7 is not substantiallyaltered. In addition, the passband of the low-pass filter 22 is suchthat the undesirable signal components, linked to sampling bydiscrete-time reading, are attenuated by at least 20 dB.

In order to prevent offsets that could jeopardize control of theoscillations of the microelectromechanical loop 18, both the band-passfilter 21 and the low-pass filter 22 are based upon amplifiers providedwith auto-zero function.

The variable-gain amplifier 23 is of a continuous-time fullydifferential type, is cascaded to the low-pass filter 22, and hasoutputs connected to the driving terminals 13 a, 13 b of themicrostructure 2, for supplying driving voltages V_(D1), V_(D2) such asto sustain the oscillation of the microelectromechanical loop 18 at thedriving frequency ω_(D), close to the mechanical resonance frequencyω_(R) of the microstructure 2. For this purpose, the gain G of thevariable-gain amplifier 23 is determined by the controller 24 through acontrol signal V_(C) correlated to the filtered feedback voltagesV_(FB1)″, V_(FB2)″ supplied by the low-pass filter 22. The controller 24is, for example, a discrete-time PID controller.

In particular, the gain G is determined so as to maintain the conditionsof oscillation of the microelectromechanical loop 18 (unit loop gainwith phase shift that is an integer multiple of 360°). For this purpose,the controller 24 receives at input the reference voltage V_(REF), whichindicates the desired reference amplitude of oscillation.

The comparator 25 has inputs connected to the inputs of thevariable-gain amplifier 23, which define control nodes 25 a, andreceives the difference voltage ΔV between the filtered feedbackvoltages V_(FB1)″, V_(FB2)″ at output from the low-pass filter 22. Thecomparator 25 switches at each zero crossing of the difference voltageΔV, thus operating as frequency-detector device. In one embodiment, thecomparator 25 is connected to a single control node and switches at eachzero crossing of one of the filtered feedback voltages V_(FB1)″,V_(FB2)″ (the zero crossings of the filtered feedback voltages V_(FB1)″,V_(FB2)″ and of the difference voltage ΔV coincide).

The output of the comparator 25, which is connected to an input of thephase-shifter module 26, supplies a native clock signal CK_(N) havingthe current frequency of oscillation of the microelectromechanical loop18. The native clock signal CK_(N) is, however, phase-shifted withrespect to the driving mass, on account of the presence of the chargeamplifier 20, of the band-pass filter 21, and of the low-pass filter 22.

The phase-shifter module 26 supplies a quadrature clock signal CK₉₀,which is phase-shifted by 90° with respect to the native clock signalCK_(N) and is used for timing the controller 24. In practice, thequadrature clock signal CK₉₀ switches at the maxima and at the minima ofthe filtered feedback voltages V_(FB1)″, V_(FB2)″ at output from thelow-pass filter 22. The controller 24 is thus properly timed so as todetect the peak values of the difference ΔV between the filteredfeedback voltages V_(FB1)″, V_(FB2)″.

As has been mentioned, the oscillator 27, the temperature-compensationmodule 28, the synchronization stage 29, and the timing generator 30co-operate for supplying timing signals for the driving device 3, forthe read generator 4, and for the read device 5.

In greater detail, the oscillator 27 generates a master clock signalCK_(M) and an auxiliary clock signal CK_(AUX), in a way asynchronouswith respect to the oscillations of the microelectromechanical loop 18and, in particular, of the driving mass 7. The oscillator 27 iscalibrated in the factory so that a master frequency ω_(M) of the masterclock signal CK_(M) is equal to an integer multiple N, preferably apower of 2, of the driving frequency ω_(D) of the microelectromechanicalloop 18 (ω_(M)=Nω_(D), with N greater than 2⁹, for example equal to2¹⁰). For this purpose, a frequency-calibration register 31 isassociated to the oscillator 27 and enables modification from outside ofthe frequency of the master clock signal CK_(M).

The auxiliary clock signal CK_(AUX), generated by frequency divisionfrom the master clock signal CK_(M), has an auxiliary frequency w_(AUX)equal to ω_(M)/N, i.e., substantially equal to the driving frequencyω_(D).

The temperature-compensation module 28 comprises a temperature sensor 28a and a memory 28 b. The temperature sensor 28 a is integrated in thesame semiconductor chip as the oscillator 27 and is thermally coupledthereto. The memory 28 b, the contents of which are defined duringcalibration in the factory, comprises an empirical table of correctionvalues of the frequency as a function of the temperature. In use, thememory 28 b returns a correction value Δ(in response to a temperaturevalue T supplied by the temperature sensor 28 a. The correction valueextracted is sent to the oscillator 27 and used for compensating thefrequency drifts caused by temperature variations, regarding both themechanical part (microstructure 2) and the electronics (oscillator 27).

The synchronization stage 29 receives the native clock signal CK_(N)from the comparator 25 and the auxiliary clock signal CK_(AUX) from theoscillator 27 and is configured to generate a base clock signal CK_(B)according to a first operating mode at steady-state and to a secondoperating mode in stabilization transients. The stabilization transientsoccur at start-up (turning-on or exit from a low-consumption, orpower-down, configuration), or else when the oscillations of the drivingmass 7 are disturbed as a result of external shocks.

As explained in detail hereinafter, in steady-state conditions the baseclock signal CK_(B) is equal to the native clock signal CK_(N),translated in phase by an amount such as to compensate the phase shift(in advance or in delay) introduced by the components of theelectromechanical loop 18 arranged between the driving mass 7 and thevariable-gain amplifier 23 (i.e., the charge amplifier 20, the band-passfilter 21, and the low-pass filter 22, in the embodiment described).Hence, in steady-state conditions, the base clock signal CK_(B) has thedriving frequency ω_(D) and is in phase with the oscillations of thedriving mass 7.

In the stabilization transients, instead, the base clock signal CK_(B)has the same frequency as the auxiliary clock signal CK_(AUX). In oneembodiment, the base clock signal CK_(B) is also phase-shifted by theamount used for the compensation in steady-state conditions. In effect,in the stabilization transients, the phase is substantially of noeffect, and the phase re-alignment may not be carried out. In this case,the base clock signal CK_(B) is synchronous in frequency and phase withthe auxiliary clock signal CK_(AUX).

The synchronization stage 29 switches from the second operating mode(transient operation) to the first operating mode (steady-stateoperation) when the frequency of the native clock signal CK_(N) (i.e.,the current frequency of oscillation of the microelectromechanical loop18) is close to the driving frequency ω_(D). In order to verify theswitching condition, the synchronization stage 29 compares the frequencyof the native clock signal CK_(N) with the frequency of the auxiliaryclock signal CK_(AUX), which is calibrated at the driving frequencyω_(D). When the difference is lower than a threshold, thesynchronization stage 29 passes from the second operating mode to thefirst operating mode. Instead, if the difference increases above thethreshold, the synchronization stage 29 returns to the second operatingmode.

The timing generator 30 receives the master clock signal CK_(M) from theoscillator 27 and the base clock signal CK_(B) from the synchronizationstage 29 and uses them for generating the timing signals necessary forthe discrete-time components and, more in general, for proper operationof the gyroscope 1.

In particular, the timing generator 30 supplies a first timing signalΦ₁, a second timing signal Φ₂, and a third timing signal Φ₃, which havesensing frequency equal to an integer multiple of the frequency of thebase clock signal CK_(B) (which, in steady-state conditions, coincideswith the driving frequency ω_(D); for example, in one embodiment, theinteger multiple is 40).

The read generator 4 is timed by the third timing signal Φ₃ and suppliesto the driving mass 7 and to the sensing mass 8 a square-wave readsignal V_(R) with rising and falling edges, respectively, at the startand at the end of the sensing step of each cycle (in practice coincidingwith the edges of the third timing signal Φ₃). In one embodiment, theread signal V_(R) is a voltage that varies between 0 V and 2V_(CM),where V_(CM) is a common-mode voltage for the components of themicroelectromechanical loop 18.

The temporal correlation between the read signal V_(R) and the timingsignals Φ₁, Φ₂, Φ₃ is illustrated in FIG. 4 and is defined to implementsensing and control cycles according to the correlated-double-sampling(CDS) technique. The first and second timing signals Φ₁, Φ₂ are high ina first fraction (t₀-t₁) of each cycle (approximately one fifth of theperiod, reset step), whereas the third timing signal Φ₃ is low. Then(instant t₁), the first timing signal Φ₁ switches and the situationremains unvaried for a second fraction (t₁-t₂) of the period(approximately two fifths, offset-sampling step; by “offset” is meant,here and in what follows, both the static offset and the contributionsof flicker noise associated to the various components). At an instantt₂, the second timing signal Φ₂ switches and remains stable during thethird and last fraction (t₂-t₃) of the period (once again two fifths,sensing step).

The read device 5 is of the discrete-time open-loop type and, in theembodiment described herein, is configured to execute a so-called“double-ended” reading of the displacements of the sensing mass 8according to the respective degree of freedom (in particular, fordetecting a position y of the sensing mass along the sensing axis Y). Inparticular, the read device 5 has inputs connected to the signal-sensingterminals 17 a, 17 b of the microstructure 2 and an output 5 a, whichsupplies an output signal S_(OUT), representative of the angularvelocity (of the microstructure 2.

As shown in FIG. 5, in one embodiment the read device 5 comprises acharge amplifier 32, a demodulator 33, which receives the master clocksignal CK_(M) and the base clock signal CK_(B), a sample-and-hold (S&H)stage 34, a low-pass filter 35, and an output amplifier 36, which arecascaded to one another. The charge amplifier 32 and the demodulator 33are of the switched-capacitor fully differential type.

The gyroscope 1 operates as hereinafter described. In steady-stateconditions, the driving mass 7 is kept in oscillation along the drivingaxis X by the driving device 3 at the driving frequency ω_(D) and withcontrolled amplitude. The sensing mass 8 is driven in motion along thedriving axis X by the driving mass 7. Consequently, when themicrostructure 2 rotates about a gyroscopic axis perpendicular to theplane of the axes X, Y at a certain instantaneous angular velocity ω,the sensing mass 8 is subject to a Coriolis force, which is parallel tothe sensing axis Y and is proportional to the angular velocity (of themicrostructure 2 and to the velocity of the two masses 7, 8 along thedriving axis X. More precisely, the Coriolis force (F_(C)) is given bythe following equation:

F_(C)=2M_(S)ωx′

where M_(S) is the value of the sensing mass 8, (is the angular velocityof the microstructure 2, and x′ is the velocity of the two masses 7, 8along the driving axis X. In steady-state conditions, the velocity x′varies in a sinusoidal way at the driving frequency ω_(D), with a phaseshift of 90° with respect to the position x according to the drivingaxis X and with amplitude substantially constant as the temperaturevaries (the variations are ordinarily less than 1%). The displacementsof the sensing mass 8 caused by the Coriolis force are read by applyingthe read signal V_(R) to the sensing mass 8 itself and convertingdifferential charge packets thus produced into the output signalS_(OUT), by means of the read device 5.

The controller 24, the comparator 25, and the phase-shifter module 26co-operate with the band-pass filter 21, the low-pass filter 22, and thevariable-gain amplifier 23 for creating and maintaining the conditionsof oscillation of the microelectromechanical loop 18 in differentoperating steps of the gyroscope 1.

In particular, in the stabilization transients the oscillations of thedriving mass 7 and, consequently, the signals present in themicroelectromechanical loop 18 do not have a sufficient amplitude toenable proper detection of the zero crossing of the difference voltageΔV. The comparator 25 switches in an incoherent way with respect to theoscillations of the driving mass 7, because the noise contributionprevails over the useful signal. Consequently, the synchronization stage29 operates in the second operating mode, and the base clock signalCK_(B) has the same frequency as the auxiliary clock signal CK_(AUX)(ω_(M)/N). The second operating mode of the synchronization stage 29guarantees co-ordinated timing of the steps of reading of the positionof the driving mass 7 and of the sensing mass 8 even when theoscillations of the microelectromechanical loop 18 are not sufficientlylarge to enable the comparator 25 to detect the frequency correctly.

When the amplitude of the oscillations of the driving mass 7 approachesthe steady-state conditions, the amplitude of the difference voltage ΔVincreases and the weight of the noise contribution is reducedaccordingly. The recognition of the zero crossing of the differencevoltage ΔV tends to become progressively more precise, until thesynchronization stage 29 detects that the frequency of the native clocksignal CK_(N) supplied by the comparator 25 is close to the frequency ofthe auxiliary clock signal CK_(AUX) and hence to the driving frequencyω_(D). The synchronization stage 29 switches to the first operatingmode, and thus the base clock signal CK_(B) is synchronous in frequencyand phase with the oscillations of the driving mass 7.

The synchronism is hence maintained until a power-off command or anenergy-save (power-down or sleep-mode) command or else an external event(shock) intervenes.

The solution described enables proper actuation of the gyroscope 1, inparticular as regards timing of the discrete-time components, andreading of the displacements of the driving mass 7 and of the sensingmass 8. The synchronization is achieved without the use ofphase-locked-loop (PLL) circuits, with considerable saving in terms ofarea occupation, consumption, and additional components external to thechip.

According to one embodiment, illustrated in FIG. 6, the synchronizationstage 29 comprises a clock-verify module 40, a multiplexer 41, and aphase-alignment module 42, associated to which is a phase-calibrationregister 43.

The clock-verify module 40 is connected to the comparator 25 and to theoscillator 27 for receiving the native clock signal CK_(N) and theauxiliary clock signal CK_(AUX) on respective comparison inputs 40 a, 40b, and the master clock signal CK_(M) on a clock input. In addition, theclock-verify module 40 is structured to verify the permanence of thecurrent frequency of the native clock signal CK_(N) in an admissibilityrange around the driving frequency ω_(D). The clock-verify module 40supplies a clock lock signal CK_LOCK that has a lock (logic) value, whenthe current frequency of the native clock signal CK_(N) falls within theadmissibility range and an asynchronous-frequency (logic) value in theopposite case.

The multiplexer 41 has signal inputs 41 a, 41 b respectively connectedto the comparator 25 and to the oscillator 27 for receiving the nativeclock signal CK_(N) and the auxiliary clock signal CK_(AUX), and acontrol input 41 c, connected to the output of the clock-verify module40 for receiving the clock lock signal CK_LOCK. The multiplexer 41moreover has an output connected to the phase-alignment module 42 andtransfers at output the native clock signal CK_(N) when the clock locksignal CK_LOCK has the lock value, and the auxiliary clock signalCK_(AUX) when the clock lock signal CK_LOCK has theasynchronous-frequency value.

The phase-alignment module 42, which receives also the master clocksignal CK_(M) from the oscillator 27, applies a calibrated phase shiftΔ(to the clock signal received from the multiplexer 41. The calibratedphase shift is stored in the phase-calibration register 43 duringcalibration in the factory and is substantially equal, in absolutevalue, to the phase shift introduced by the components of themicroelectromechanical loop arranged between the driving mass 7 and thevariable-gain amplifier 23 (i.e., the charge amplifier 20, the band-passfilter 21, and the low-pass filter 22, in the embodiment described).

Said phase shift is hence eliminated, and the base clock signal CK_(B)is in phase with the oscillations of the driving mass 7 according to thedriving axis X.

The output of the phase-alignment module 42 is connected to the timinggenerator 30 for supplying the base clock signal CK_(B).

FIG. 7 shows in greater detail the clock-verify module 40, whichcomprises a first clock counter 45, a second clock counter 46, anenabling element 47 and a count comparator 48.

The first clock counter 45 has a count input (defined by the secondinput 40 b) coupled to the oscillator 27 for receiving the auxiliaryclock signal CK_(AUX) and stores a first count value C₁.

In addition, the first clock counter 45 is provided with asynchronization logic network 49, which generates a synchronizationsignal S_(SYNC). The synchronization signal S_(SYNC) is supplied to theenabling element 47 and to the count comparator 48 and has an enablingvalue when the first count value C₁ stored in the first clock counter 45is lower than a control value C₁*. When the control value C₁* isreached, the synchronization signal S_(SYNC) switches to a disablingvalue and, moreover, the first clock counter 45 is reset.

The second clock counter 46 has a count input (defined by the firstinput 40 a) coupled to the comparator 25, for receiving the native clocksignal CK_(N), and an enabling input connected to the enabling element47. The second clock counter 46 stores a second count value C₂, which isincremented at each cycle of the native clock signal CK_(N) when thesecond clock counter 46 is enabled.

The enabling element 47 is, for example, a flip-flop of a DT type andreceives the synchronization signal S_(SYNC) on a data input from thefirst clock counter 45 and the native clock signal CK_(N) on a clockinput from the comparator 125. In this way, the enabling element 47transfers the value of the synchronization signal S_(SYNC) to theenabling input of the second clock counter 46, which is thus incrementedat each cycle of the native clock signal CK_(N), as long as thesynchronization signal S_(SYNC) remains at the enabling value (i.e.,until the first clock counter 45 reaches the control value C₁*). Thenative clock signal CK_(N) on the clock input of the enabling element 47prevents spurious switchings and errors of the second clock counter 46.

The count comparator 48 is coupled to the first clock counter 45, fromwhich it receives also the synchronization signal S_(SYNC), and to thesecond clock counter 46.

The count comparator 48 supplies at output the clock lock signal CK_LOCKand determines the value thereof as described hereinafter.

When the first clock counter 45 starts a count after having been reset,the synchronization signal S_(SYNC) switches to the enabling value (asshown in FIG. 8, which regards a start-up step in which the driving massis forced into oscillation starting from a resting condition). Thesecond clock counter 46 is enabled and is incremented at each cycle ofthe native clock signal CK_(N), independently of the first clock counter45. In addition, the current frequency of the native clock signal CK_(N)progressively approaches the frequency of the auxiliary clock signalCK_(AUX), as a result of the increase in amplitude of the oscillationsdue to forcing. The frequency variations of the native clock signalCK_(N) are illustrated in an exaggerated way in FIG. 8.

When the first clock counter 45 reaches the control value C₁*, thesynchronization signal S_(SYNC) switches to the disabling value, and thefinal count value C₂* stored in the second clock counter 46 is frozen.

In addition, the count comparator 48 takes the control value C₁* and thefinal count value C₂*, respectively, from the first clock counter 45 andfrom the second clock counter 46 and assigns a value to the clock locksignal CK_LOCK according to whether the lock condition:

${\frac{C_{1}^{*} - C_{2}^{*}}{C_{1}^{*}}} \leq X$

is verified or not.

More precisely, if the control value C₁* and the final count value C₂*satisfy the lock condition, assigned to the clock lock signal CK_LOCK isthe lock value. In this case, in fact, the current frequency of thenative clock signal CK_(N) is close to the frequency of the auxiliaryclock signal CK_(AUX) and hence to the driving frequency ω_(D).

In the opposite case, i.e., if the inequality is not verified, the countcomparator 48 assigns to the clock lock signal CK_LOCK theasynchronous-frequency value.

FIG. 9 illustrates a portion of an electronic system 100 according toone embodiment of the present disclosure. The system 100 incorporatesthe gyroscope 1 and may be used in devices, such as, for example, apalmtop computer (personal digital assistant, PDA), a laptop or portablecomputer, possibly with wireless capacity, a cell phone, a messagingdevice, a digital music reader, a digital camera, or other devicesdesigned to process, store, transmit or receive information. Forexample, the gyroscope 1 may be used in a digital camera for detectingmovements and carrying out an image stabilization. In other embodiments,the gyroscope 1 is included in a portable computer, a PDA, or a cellphone for detecting a free-fall condition and activating a safetyconfiguration. In a further embodiment, the gyroscope 1 is included in auser interface activated by movement for computers or consoles forvideogames. In a further embodiment, the gyroscope 1 is incorporated ina satellite-navigation device and is used for temporary tracking ofposition in the case of loss of the satellite-positioning signal.

The electronic system 100 may comprise a controller 110, an input/output(I/O) device 120 (for example, a keyboard or a screen), the gyroscope 1,a wireless interface 140, and a memory 160, of a volatile or nonvolatiletype, coupled to one another through a bus 150. In one embodiment, abattery 180 may be used for supplying the system 100. It is to be notedthat the scope of the present disclosure is not limited to embodimentshaving necessarily one or all of the devices listed.

The controller 110 may comprise, for example, one or moremicroprocessors, microcontrollers, and the like.

The I/O device 120 may be used for generating a message. The system 100may use the wireless interface 140 for transmitting and receivingmessages to and from a wireless-communications network with aradiofrequency (RF) signal. Examples of wireless interface may comprisean antenna, a wireless transceiver, such as a dipole antenna, eventhough the scope of the present disclosure is not limited from thisstandpoint. In addition, the I/O device 120 may supply a voltagerepresenting what is stored either in the form digital output (ifdigital information has been stored) or in the form analog output (ifanalog information has been stored).

Finally, it is clear that modifications and variations may be made tothe gyroscope and to the method described, without thereby departingfrom the scope of the present disclosure.

In particular, the gyroscope could have any different micromechanicalstructure. For example, the disclosure can be advantageously exploitedin: gyroscopes with one or more sensing masses linearly movable withrespect to the driving mass and sensitive to rotations of pitch and/orroll (in addition to rotations of yaw); gyroscopes with cantileversensing masses or sensing masses in the form of beams oscillating aboutcentroidal or non-centroidal axes; and uniaxial and multiaxialgyroscopes with angularly oscillating driving mass.

The various embodiments described above can be combined to providefurther embodiments. All of the U.S. patents, U.S. patent applicationpublications, U.S. patent application, foreign patents, foreign patentapplication and non-patent publications referred to in thisspecification are incorporated herein by reference, in their entirety.Aspects of the embodiments can be modified, if necessary to employconcepts of the various patents, application and publications to provideyet further embodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

1. A microelectromechanical gyroscope comprising: a body; a driving massmovable with respect to the body with a first degree of freedomaccording to a driving axis; a sensing mass mechanically coupled to thedriving mass so as to be drawn in motion according to the driving axisand movable with respect to the driving mass with a second degree offreedom according to a sensing axis, in response to rotations of thebody; a driving device, forming a microelectromechanical control loopwith the body and the driving mass and configured to maintain thedriving mass in oscillation according to the driving axis with a drivingfrequency, the driving device including: a frequency detector coupled toa control node of the microelectromechanical control loop and configuredto provide a first clock signal with a frequency equal to a currentoscillation frequency of the driving mass; a synchronization stagecoupled to the frequency detector and configured to apply, in a firstoperation mode, a calibrated phase shift to the first clock signal tocompensate for a phase shift arising between the driving mass and thecontrol node.
 2. A gyroscope according to claim 1, wherein thesynchronization stage is configured to provide a second clock signalthat, in the first operation mode, has the same frequency as the firstclock signal and a phase shift equal to the calibrated phase shift.
 3. Agyroscope according to claim 2, comprising a timing generator coupled tothe synchronization stage and configured to provide a plurality oftiming signals based on the second clock signal.
 4. A gyroscopeaccording to claim 3, wherein the timing signals have frequency equal toan integer multiple of the frequency of the second clock signal.
 5. Agyroscope according to claim 3, wherein the microelectromechanicalcontrol loop comprises a switched-capacitor component configured to beoperated through the timing signals.
 6. A gyroscope according to claim3, comprising a reading generator coupled to the driving mass and to thesensing mass and configured to provide a square-wave reading signal tothe driving and sensing masses.
 7. A gyroscope according to claim 6,wherein the reading generator is coupled to the timing generator and isconfigured to provide the square-wave reading signal based on at leastone of the timing signals.
 8. A gyroscope according to claim 2,comprising an oscillator configured to provide to the synchronizationstage a third clock signal having an auxiliary frequency close to thedriving frequency.
 9. A gyroscope according to claim 8, wherein thesynchronization stage is configured, in a second operation mode, toprovide the second clock signal with a frequency equal to the auxiliaryfrequency of the third clock signal.
 10. A gyroscope according to claim9, wherein the synchronization stage is configured to compare thefrequency of the first clock signal with the auxiliary frequency of thethird clock signal, switch from the second operation mode to the firstoperation mode when a difference between the frequency of the firstclock signal and the auxiliary frequency of the third clock signal fallsbelow a threshold, and switch from the first operation mode to thesecond operation mode when the difference between the frequency of thefirst clock signal and the auxiliary frequency of the third clock signalexceeds the threshold.
 11. A gyroscope according to claim 9, wherein thesynchronization stage comprises: a clock verify module coupled to thefrequency detector and to the oscillator and configured to receive thefirst clock signal and the third clock signal, and provide a clock locksignal having a first logic value, when the frequency of the first clocksignal is within an acceptability range around the driving frequency,and a second logic value, when the frequency of the first clock signalis out of the acceptability range; a selector coupled to the frequencydetector module and to the oscillator and configured to receive thefirst clock signal and the third clock signal, and configured, undercontrol by the clock verify module, to output the first clock signalwhen the clock lock signal has the first logic value, and output thethird clock signal when the clock lock signal has the second logicvalue; and a phase alignment module coupled to the selector andconfigured to apply the calibrated phase shift to the first clock signalwhen the clock lock signal has the first logic value.
 12. A gyroscopeaccording to claim 8, comprising a temperature compensation modulecoupled to the oscillator and configured to cause the oscillator tocompensate for frequency drifts caused by temperature variations.
 13. Agyroscope according to claim 12, wherein the temperature compensationmodule comprises a temperature sensor thermally coupled to theoscillator, and a memory configured to store a plurality of correctionvalues and provide to the oscillator respective correction values inresponse to temperature values provided by the temperature sensor.
 14. Agyroscope according to claim 12, wherein the driving device includes: anamplifier coupled to the driving mass and configured to produce adetection signal based on detected motion of the driving mass; and afilter configured to filter the detection signal and produce the phaseshift which the synchronization stage is configured to compensate for.15. A system, comprising: a control unit; and a microelectromechanicalgyroscope coupled to the control unit and including: a body; a drivingmass movable with respect to the body with a first degree of freedomaccording to a driving axis; a sensing mass mechanically coupled to thedriving mass so as to be drawn in motion according to the driving axisand movable with respect to the driving mass with a second degree offreedom according to a sensing axis, in response to rotations of thebody; a driving device, forming a microelectromechanical control loopwith the body and the driving mass and configured to maintain thedriving mass in oscillation according to the driving axis with a drivingfrequency, the driving device including: a frequency detector coupled toa control node of the microelectromechanical control loop and configuredto provide a first clock signal with a frequency substantially equal toa current oscillation frequency of the driving mass; a synchronizationstage coupled to the frequency detector and configured to apply, in afirst operation mode, a calibrated phase shift to the first clock signalto compensate for a phase shift arising between the driving mass and thecontrol node.
 16. A system according to claim 15, wherein: thesynchronization stage is configured to provide a second clock signalthat, in the first operation mode, has the same frequency as the firstclock signal and a phase shift equal to the calibrated phase shift; andthe driving device includes a timing generator coupled to thesynchronization stage and configured to provide a plurality of timingsignals based on the second clock signal.
 17. A system according toclaim 16, wherein: the driving device includes an oscillator configuredto provide to the synchronization stage a third clock signal having anauxiliary frequency close to the driving frequency; the synchronizationstage is configured, in a second operation mode, to provide the secondclock signal with a frequency equal to the auxiliary frequency of thethird clock signal; the synchronization stage is configured to comparethe frequency of the first clock signal with the auxiliary frequency ofthe third clock signal, switch from the second operation mode to thefirst operation mode when a difference between the frequency of thefirst clock signal and the auxiliary frequency of the third clock signalfalls below a threshold, and switch from the first operation mode to thesecond operation mode when the difference between the frequency of thefirst clock signal and the auxiliary frequency of the third clock signalexceeds the threshold.
 18. A system according to claim 16, wherein thedriving device includes an oscillator configured to provide to thesynchronization stage a third clock signal having an auxiliary frequencyclose to the driving frequency and the synchronization stage comprises:a clock verify module coupled to the frequency detector and to theoscillator and configured to receive the first clock signal and thethird clock signal, and provide a clock lock signal having a first logicvalue, when the frequency of the first clock signal is within anacceptability range around the driving frequency, and a second logicvalue, when the frequency of the first clock signal is out of theacceptability range; a selector coupled to the frequency detector moduleand to the oscillator and configured to receive the first clock signaland the third clock signal, and configured, under control by the clockverify module, to output the first clock signal when the clock locksignal has the first logic value, and output the third clock signal whenthe clock lock signal has the second logic value; and a phase alignmentmodule coupled to the selector and configured to apply the calibratedphase shift to the first clock signal when the clock lock signal has thefirst logic value.
 19. A method for actuating a microelectromechanicalgyroscope that includes a body, a driving mass movable with respect tothe body with a first degree of freedom according to a driving axis, anda sensing mass mechanically coupled to the driving mass so as to bedrawn in motion according to the driving axis and movable with respectto the driving mass with a second degree of freedom according to asensing axis, in response to rotations of the body, the methodcomprising: maintaining the driving mass in oscillation according to thedriving axis with a driving frequency through a microelectromechanicalcontrol loop including the body and the driving mass; detecting acurrent oscillation frequency of the driving mass at a control node ofthe microelectromechanical control loop; applying, in a first operationmode, a calibrated phase shift to the first clock signal calibrated tocompensate for a phase shift caused by components of themicroelectromechanical control loop arranged between the driving massand the control node.
 20. A method according to claim 19, comprising:producing a second clock signal that, in the first operation mode, hasthe same frequency as the first clock signal and a phase shift equal tothe calibrated phase shift; producing a third clock signal having afrequency close to the driving frequency; comparing the frequency of thefirst clock signal with the frequency of the third clock signal;providing, in a second operation mode, the second clock signal with afrequency equal to the frequency of the third clock signal; switchingfrom the first operation mode to the second operation mode when adifference between the frequency of the first clock signal and thefrequency of the third clock signal exceeds a threshold stage, wherein;and switching from the second operation mode to the first operation modewhen the difference between the frequency of the first clock signal andthe auxiliary frequency of the third clock signal falls below thethreshold.